The present invention relates to a telescopic rod antenna apparatus mounted on an automobile and the like and, more particularly, to a telescopic rod antenna apparatus having a waterproof means for preventing rainwater or the like from soaking into the antenna apparatus through a gap between an antenna element and an opening of an antenna holding tube.
Generally, in a prior art telescopic rod antenna apparatus, a telescopic antenna element, which is constituted by slidably coupling a plurality of rods of conductive tube members having different diameters, is insertably held in an antenna holding tube. The rod antenna apparatus so constituted has a gap between the antenna element and the opening of the antenna holding tube, a gap in a joint between a small-diameter rod and a large-diameter rod both constituting the telescopic antenna element, etc. It is thus likely that rainwater, washing, muddy water, etc. will soak into the antenna apparatus through such a gap.
Conventionally various measures have been taken against water entering the telescopic rod antenna apparatus. However, at present, there is no telescopic rod antenna apparatus capable of completely preventing water from soaking thereinto.
In arctic weather of winter, there are many cases where water is frozen in a telescopic rod antenna apparatus to make it impossible to extend an antenna by a driving motor.
The prior art telescopic rod antenna apparatus was inspected to find portions through which water such as rainwater soaks into the apparatus and know the reason why the water soaks into the apparatus. The following are results of the inspection.
(1) An antenna element constituted of a plurality of rods having different diameters and coupled slidably to each other is designed as follows. The retraction thereof is completed such that the end portion of a small-diameter rod is protruded a little more than that of a large-diameter rod in order to prevent a large gap from being formed in a joint between the rods. The end portion of the retracted antenna element looks like a roof portion of a multi-stored pagoda and contains the plural rods of different diameters arranged in tiers. A so-called top portion is provided at the tip portion of the smallest-diameter rod which is the top of the antenna element. Generally the top portion is so designed that its outside diameter is equal to that of the largest-diameter rod.
The telescopic rod antenna apparatus capable of insertably holding the above telescopic antenna element in a holding tube, has the following two types. One type is that an antenna element having rods in tiers is held in a holding tube such that its tip portion is projected from the holding tube. The other is that an antenna element is held in a holding tube almost completely such that its tip portion is buried in the holding tube and its top portion is located at the opening of the holding tube. Hereinafter the former is called a top-portion projected type, and the latter is called a top-portion buried type.
In the telescopic rod antenna apparatus of the top-portion projected type, when the antenna element is held in the holding tube, the largest-diameter rod is inserted into the opening of the holding tube with almost no gap therebetween. Thus, it is not so likely that rainwater will enter the holding tube through the opening of the tube. Since, however, the antenna element is held in the holding tube incompletely, the tip portion or the top portion of the antenna element, projected from the holding tube, is likely to contact an obstacle and break.
The telescopic rod antenna apparatus of the top-portion buried type does not cause the drawback of the projected type apparatus since the antenna element is held in the holding tube almost completely. Moreover, the top portion of the antenna element is inserted into the opening of the holding tube with almost no gap therebetween, so that it is not so likely that rainwater will enter the holding tube through the opening of the tube. However, as described below, there is a possibility that rainwater or the like will relatively easily soak into the holding tube when the antenna element is extended or retracted.
If the smallest-diameter rod is projected from the holding tube when the antenna element is extended, a great gap will appear between the inner surface of the opening of the holding tube and the tip portion of the antenna element of rods in tiers during a period of time (which corresponds to about 80% of the time required for extending the antenna element completely) from when the top portion goes out of the opening of the holding tube until the largest-diameter rod is projected from the holding tube.
It is thus likely that rainwater will easily soak into the holding tube through the great gap. Even when the antenna element is retracted, if the largest-diameter rod is held earlier than the small-diameter rod (which will occur in a conventional telescopic rod antenna apparatus), a gap is caused during a period of time from when the largest-diameter rod is held in the holding tube until the smallest-diameter rod is held therein; therefore, rainwater is likely to soak into the tube.
(2) Since it has been thought that the phenomenon of (1) cannot be prevented completely, no specific waterproof means has been provided at the opening of the holding tube. Thus, water soaking into the holding tube is drained out of a drain provided at the lower end of the holding tube.
In the telescopic rod antenna apparatus of the top-portion buried type, even though the antenna element is held completely in the holding tube, rainwater is likely to soak into the tube through a gap between the top portion and the opening, since the holding tube has no specific waterproof means at the opening. In the telescopic rod antenna apparatus of the top-portion projected type, even though the antenna element is held completely in the holding tube, rainwater is likely to soak into the tube through a gap between the largest-diameter rod and the opening for the same reason described above.
(3) Of the rods constituting the antenna element of the telescopic rod antenna apparatus, the smallest-diameter rod has a so-called, slightly roundish top portion at the tip portion. This top portion is attached to the tip portion of the smallest-diameter rod as follows. A columnar portion protruded from the lower end of the top portion is pressed into a hollow of the tip portion of the smallest-diameter rod and then the outer surface of the tip portion of the rod is caulked with a ring at two points. The ring-caulking causes a slight irregularity or distortion on the outer surface of the smallest-diameter rod. If the irregularity or distortion is relatively great, a gap will occur between the outer surface of the smallest-diameter rod and the inner surface of the subsequent rod when the antenna element is retracted, and rainwater is likely to soak into the rods through the gap.
(4) It is likely that a very small amount of rainwater will enter the rods through a gap in a joint between the smallest-diameter rod and the large-diameter rod constituting the telescopic antenna element.
As described above, the conventional telescopic rod antenna apparatus has a problem in which rainwater or the like soaks into the antenna through a gap between the end portion of the antenna element and the opening of the holding tube when the antenna element is extended or retracted in the top-portion buried type antenna apparatus, a gap between the top portion and the opening of the holding tube when the antenna element is held in the tube in the top-portion buried type antenna apparatus, a gap between the largest-diameter rod and the opening of the holding tube when the antenna element is held in the tube in the top-portion projected type antenna apparatus, a gap between the smallest-diameter rod and the subsequent rod constituting the telescopic antenna element, a gap in the joint between the smallest-diameter rod and the largest-diameter rod, etc, thereby to cause various problems.